An implantable cardioverter-defibrillator, commonly referred to as an “ICD,” is capable of recognizing tachycardia or fibrillation and delivering electrical therapy to terminate such arrhythmias. ICDs are often configured to perform pacemaking functions as well. A pacemaker generally delivers rhythmic electrical pulses to the heart to maintain a normal rhythm in patients having conduction abnormalities or bradycardia, which is too slow of heart rate. Pathologic tachycardia, which is a rapid heart rate not associated with a normal physiologic response such as a response to exercise, is typically treated with low to moderate-energy shocking pulses. The treatment of tachycardia is often referred to as “cardioversion.” Fibrillation is characterized by rapid, unsynchronized depolarizations of the myocardial tissue. Ventricular fibrillation is most often fatal if not treated within a few minutes of its onset. The termination of fibrillation, referred to as “defibrillation,” is accomplished by delivering high-energy shocking pulses.
Upon detection of fibrillation, a defibrillation therapy, referred to herein as a “regimen,” delivered by an implantable defibrillator may include delivery of multiple defibrillation waveforms. Each waveform is defined by a number of parameters including the shape and energy of each pulse. A conventional wave shape is a biphasic waveform in which two pulses that have opposite polarity are generated on the order of 100 microseconds apart. Each waveform within a regimen is delivered on the order of 10 seconds apart. During the time between each defibrillation waveform, the capacitor used for delivering the next waveform is charged, and the defibrillator re-determines if fibrillation is still present. If fibrillation is no longer detected, the regimen is terminated prior to delivering another shock.
Early implantable defibrillation systems required a thoracotomy to allow placement of electrode patches on the epicardial surface of the heart. The risk of morbidity and mortality associated with an open thoracic approach led to the development of transvenous systems that are available today. Transvenous systems include placement of a lead in the right side of the heart with an electrode in the right ventricle, typically near the apex, and a second proximal electrode, typically in the superior vena cava. However, defibrillation using a single lead in the right side of the heart is not successful in all patients and implantation of an epicardial patch is commonly indicated.
The relatively large physical size of early implantable defibrillators, due to large capacitors needed for delivering the high-energy shocks, restricted the implantation of the device to the abdominal region. As capacitor technology has improved, the size of the defibrillators has decreased making pectoral implantation feasible. With the ability to implant the device in the pectoral region, the housing of the device becomes available as an active electrode, sometimes referred to as an “active can,” in combination with the right ventricular lead eliminating the need for an epicardial patch electrode in most patients. Thus, the pectoral implantation of the device overcame the need for a thoracic approach.
Implantable defibrillation systems have been described that use either single or dual defibrillation pathways utilizing combinations of two or three electrodes, selected from a right ventricular lead and the active can. Investigations have been made to determine the optimal defibrillation electrode configuration and results show improved effectiveness of active can configurations, particularly with dual pathway defibrillation using three electrodes.
As the device size continues to be reduced, however, the effectiveness of active can configurations comes into question. Development of coronary sinus electrodes, implanted endovascularly in the area of the left heart, provides additional electrode configurations available for defibrillation. With new configurations available between electrodes implanted in the right ventricle and endovascular electrodes on the left side of the heart, investigation continues for determining the optimal electrode configuration for achieving successful defibrillation at the lowest energy requirement.
However, no single defibrillation electrode configuration will be optimal for all patients. Differences in implant location, patient anatomy and disease state, which can change overtime, will result in different optimal electrode configurations between patients and perhaps within the same patient over time. A given defibrillation pathway selected as the primary pathway based on clinical testing may not continue to be the optimal defibrillation pathway. Therefore, a final determination of an optimal electrode configuration remains elusive.
The use of multiple single or dual pathways during a defibrillation regimen, therefore, would be advantageous in patients who are not successfully treated by the first defibrillation shock waveform delivered along a primary pathway. An implantable defibrillation device is needed, therefore, which allows automatic switching between defibrillation pathways during a defibrillation regimen.